Collector assembly



Dec. 25, 1956 w. c. WILEY COLLECTOR ASSEMBLY Filed July 9, 1951 E T /2P, v, o M U C M E LMC s T wmm m N M Fe m m R H N N a w I. C M M 5 w 7 5M A M l M w H mrwll 0 y; Will] 4: i 6. w m \z LMC W (a N 5 5 m we Q \z4- E 1 E H E 1 g Q M l I 2 0 H 6 M United States Patent COLLECTORASSEMBLY William C. Wiley, Detroit, Mich., assignor to Bendix AviationCorporation, Detroit, Mich., a corporation of Delaware Application July9, 1951, Serial No. 235,744 8 Claims. (Cl. 25041.9)

This invention relates to mass spectrometers and more particularly to anew collector assembly for mass spectrometers. The invention alsorelates to a method of collecting ions in a mass spectrometer.

Mass spectrometers are known in which ions of different mass areinitially subjected to a field of substantially constant force by an ionsource such that the ions of relatively light mass attain a greaterspeed than the ions of heavier mass. The ions are then directed into amagnetic field of substantially uniform intensity in a directionperpendicular to that of the magnetic field. This field causes the ionsto describe a substanitally circular path, preferably in a substantiallyhorizontal plane. After a predetermined number of complete revolutions,the ions are collected and the time required for them to describe thepredetermined number of revolutions is measured. Since the relativelylight ions travel faster than the heavier ions, the time required forthe ions to complete their travel gives an indication of their masses.

In order for the ions to describe a complete number of revolutions, theion collector must be located directly below or above the ion source.This prevents the ion collector from being as large as might be desiredand causes only the ions describing an optimum path to actually reachthe collector. Since a large number of ions are lost between the ionsource and the ion collector, the strentgh of the measurements of thetime required for the ions of ditierent mass to reach the collectorassembly is reduced, with a corresponding reduction in the accuracy ofthe measurements. Further inaccuracies are produced because some of theions not describing an optimum path travel through a number ofrevolutions different from the predetermined number and thus reach thecollector assembly at a different time than the other ions ofcorresponding mass.

This invention provides a mass spectrometer having an ion collector sodisposed that the ions are collected after they rotate through apredetermined angle which is less than a complete revolution. The ioncollector is also disposed to collect substantially all of the ionsafter they have travelled through the predetermined angular distancefrom the ion source. By so disposing the ion collector, strong andaccurate measurements of the time required for the ions to travelthrough the angular distance are obtained.

An object of this invention is to provide a mass spectrometer having anion collector for collecting the ions travelling through the massspectrometer after they have travelled through a predetermined angulardistance ditierent from an integral number of revolutions.

Another object is to provide a mass spectrometer of the above characterhaving an ion collector for collecting the ions travelling through thespectrometer after they have travelled through a predetermined angulardistance constituting less than a complete revolution.

A further object is to provide a mass spectrometer of the abovecharacter having an ion collector for collecting substantially all ofthe ions travelling through the mass spectrometer.

Still another object is to provide a mass spectrometer of the abovecharacter for providing strong and accurate 7 indications of the timerequired for the ions to travel through a predetermined angulardistance.

A still further object is to provide a method for collecting all of theions travelling through a mass spectrometer after they have described apredetermined angle less than a complete revolution.

Other objects and advantages will be apparent from a detaileddescription of the invention and from the appended drawings and claims.

In the drawings:

Figure 1 is a perspective view illustrating the disposition of a massspectrometer relative to a magnet for creating a magnetic field in thespectrometer;

Figure 2 is a perspective view of the mass spectrometer shown in Figure1, illustrating the appearance and relative disposition of an ion sourceand an ion collector in the spectrometer;

Figure 3 is a perspective view illustrating in detail the ion sourceshown in Figure 2.

Figure 4 is a perspective view illustrating in detail the ion collectorshown in Figure 2 and also showing in block form the measuring equipmentassociated with the collector; and

Figure 5 is a diagrammatic view illustrating the travel paths ofdifferent ions.

In one embodiment of the invention, a permanent magnet, generallyindicated at 10, is provided with a yoke 12 and oppositely disposed polepieces 14 and 16 vertically separated from each other by an air gap. Themagnet is adapted to supply a magnetic field of substantially uniformstrength across the air gap. A mass spectrometer, generally indicated at18, is located in the air gap between the pole pieces 14 and 16 and hasoppositely disposed cylindrical faces of a cover 20 pressing against thehorizontal surfaces of the pole pieces 14 and 16. An ion source,generally indicated at 22, and an ion collector, generally indicated at24, are suitably housed within the cover 20, which is evacuated to arelatively low pressure.

The ion source 22 includes a horizontally disposed filament 26 made froma suitable material, such as tungsten, and negatively biased withrespect to ground by a battery 59. The filament is supported by a pairof leads which extend from terminals 28 suitably secured to the cover20. A grounded shield 30 having an inverted U-shape and a relativelynarrow slot 32 in the horizontal leg of the U is suitably supported bythe cover 20 to substantially enclose the filament 26. The horizontalleg of the shield 30 supports a receptacle 34 having a horizontallydisposed slot aligned with the slot 32 and a relatively narrow slot 36in one of its vertical walls. A plate 38 having a relatively narrow slot40 aligned with a slot 36 is mechanically connected as by brackets tothe receptacle 34 in insulated relationship to the receptacle and iselectrically connected to a conventional pulse forming circuit 42,adapted to supply pulses having a negative voltage relative to ground. Aconduit 44 extends from an external container (not shown) through anopening in one of the vertical Walls in the receptacle 34. The containeris adapted to hold molecules of the difierent gases constituting anunknown mixture and to feed molecules of such gases into the massspectrometer 18 for analysis as to their masses or as to their relativeabundance.

. The ion collector 24 includes a shield 48 supported as by brackets 50within the cover 20 and a receptacle 52 supported as by brackets 54within the shield 48 in insulated relationship to the shield. The shield48 is grounded, and a relatively low positive voltage, such as 20 volts,is applied on the receptacle 52 by a battery 60 to collect secondaryelectrons emitted from the receptacle when. ions 0 impinge on it. Theshield 48 and receptacle 52 are rectangular parallelepipeds, withcorresponding sides of each parallelepiped being parallel to oneanother. Each parallelepiped is open at the side facing the flow of ionsbut the open side of the receptacle 52 is slightly recessed within theshield 48. A grounded screen grid 56 extends across the open side of theshield 48 and also covers the open side of the receptacle because of therecessed nature of the receptacle. The open side of the shield 48 isslightly smaller in area than the area formed in the spectrometer by theplane through which the open side extends. Similarly, the open side ofthe receptacle 52 is slightly smaller in area than the open side of theshield 48. The sides of the shield and receptacle parallel to the opensides extend from a position adjacent the ion source 22 at apredetermined angle to the direction in which the ions are emitted fromthe source. Thus, the ions rotate through a predetermined angle, such as315, before they strike the side of the receptacle 52 parallel to theopen side, as will be explained in detail hereafter. A time indicator,such as an oscilloscope 58, is connected to the receptacle 52 toindicate the relative times at which the ions of different mass impingeupon the receptacle 52. The oscilloscope 58 is connected to receivevoltage pulses from the pulse forming circuit 42 for initiating thehorizontal sweep of the oscilloscope at substantially the same timenegative pulses are applied to the plate 38.

In operation, electrons are emitted from the filament 26 when thefilament is sufficiently heated by an electrical current passing throughit. These electrons are attracted upwardly through the slot 32 in thebottom of the shield 30 because of the negative bias on the filament 26with respect to the grounded receptacle 34. As the electrons movethrough the confined space within the receptacle 34, they strikemolecules of gas which flow from the container (not shown) through theconduit 44 and into the receptacle as a result of the difference inpressure between the container and the mass spectrometer. The moleculesof gas which flow into the mass spectrometer include the different gasesin the unknown mixture stored within the container and have apredetermined abundance ratio to the gases in the container.

As a result of the collision between electrons and gas molecules, someof the molecules become ionized into electrons and positive ions. Theions are attracted towards the plate 38 when a voltage pulse of negativepolarity is applied on the plate 38, and these ions are emitted throughthe slot 40 in the plate 38 and the slot 36 in the receptacle 34. Thevoltage pulse on the plate 38 is cut off before the ions reach the plateso as to apply an equal force on all of the ions. Because of thenarrowness of the slots 36 and 40 and the alignment of the slots, theions are emitted in a direction substantially perpendicular to the planeof the slots. Since the voltage on the plate 38 causes it to attract allof the ions with equal force, the ions of relatively light mass attain agreater velocity than the ions of heavier mass before they pass throughthe slot 36 in the receptacle 34. All of the ions pass in a relativelysharp pulse through the slot 36 because of the negative voltage pulseapplied on the plate 38.

The ions are also emitted in a direction substantially perpendicular tothe magnetic flux lines formed between the pole pieces 14 and 16, sincethe flux lines lie in substantially the same plane as the slots 36 and40. This causes the magnetic field to exert a force upon the ions in adirection perpendicular to both the direction of the magnetic field andthe direction of ion travel, such that the ions adopt a circular path.However, the ions of light mass rotate through their circular orbitfaster than the ions of heavy mass since the angular velocity of theions is inversely proportional to their mass. This may be seen from therelationship d eH dt m where H=the strength of the magnetic field;

m=the mass of an ion; e=the electrical charge on the ion; and

The above equation is derived on pages 280 and 281 of Principles ofElectricity by Page and Adams (1931 edition).

Because of their relatively great angular velocity, the ions ofrelatively light mass reach the receptacle 52 before the ions of heavymass and produce a charge upon the receptacle when they impinge on it.Any secondary emission which takes place when ions impinge on thereceptacle 52 is collected back at the receptacle because of thedifference in voltage between the receptacle and the shield 48 and theelectrical and mechanical sheltering action provided by the receptacle.Thus, by measuring the charges appearing on the receptacle 52 and thetimes at which the charges appear, an indication can be obtained by theindicator 58 of the masses of the different gases in the unknown mixtureand the relative abundance of each gas.

The mass spectrometer disclosed above has several important advantages.Because of the relative displacement between the ion source 22 and theion collector 24, the ion collector is able to occupy subsatntially allof the transverse area perpendicular to the direction of the ion flow.As a result, practically all of the ions travelling in a circular orbitthrough the mass spectrometer are collected by the receptacle 52, sincethe receptacle 52 is almost as big as the shield 48 within which it iscontained. By collecting substantially all of the ions which describe acircular path in the mass spectrometer, the receptacle 52 producesstrong and positive signals which give a clear indication of the timerequired for the ions of different mass to reach the receptacle.

The signals from the ion collector 24 are also sharply defined becauseof the fact that all of the ions of a par ticular mass are collected atthe same time. This simultaneous collection of all of the ions of aparticular mass occurs even though the ion collector 24 is located at aparticular angle relative to the direction of ion emission, asillustrated in Figure 2. As discussed previously, it the ions of allmasses describe a circular path of similar radiusfor example, the path60 illustrated in Figure 5the ions of light mass will reach thecollector before the ions of heavy mass because of the relatively greatangular velocity imparted to them by the action of the magnetic field.

It may happen, however, that some ions of a particular mass may beejected into the magnetic field in the mass spectrometer at a slowerspeed than other ions of the same mass. Since the centripetal forceexerted upon the ions is inversely proportional to the speed of the ionsas well as directly proportional to their mass, the ions having a speedbelow the average for their mass travel through a path, such as the path62, having a smaller radius than the path 60. The relatively shortlength of the path 60 as compared to the length of the path 62compensates for the slow speed of the ions describing this path andcauses the ions describing this path to rotate through a predeterminedangular arc, such as 315, in substanitally the same time as the otherions of the same mass.

In like manner, ions of a particular path having a speed greater thanthe average speed for ions of that mass may describe a path, such as thepath 64, having a greater radius than the path 60 and may rotate througha predetermined angular distance in the same time as required for theother ions of the same mass. Thus, by disposing the ion collector 24relative to the ion source 22 so that the ions rotate through apredetermined angle, such as 315, before impinging on the ion collector,all of the ions of a particular mass are collected at the same time,regardless of their speed. The collection of all ions having aparticular mass also occurs at the same instant since all of the ionsrotate through the same angular distance, which is less than onecomplete revolution. Collecting all of the ions before one completerevolution prevents some of the ions from rotating through a differentnumber of revolutions than other ions of the same mass and minimizes anyblurring in the measurements that are obtained.

The magnetic force applied on the ions should be fairly large in orderto apply a relatively great centripetal force on the ions and to makethe diameter of the cylindrical cover 24) defining the outer dimensionsof the mass spectrometer relatively small. The magnetic force shouldalso be fairly large so that the ions can move at a relatively highspeed and still describe a circular path of reasonable diameter. Fastmovement of the ions minimizes the deflecting action produced by strayfields, either electrostatic or electromagnetic, and reduces anyblurring that may take place. Since the strength of the magnetic fieldproduced by the magnet is inversely proportional to the length of thegap between the pole pieces 14 and 16, it is desirable to keep the gapas small as possible. This is etfectuated by removing the ion collector24 from a position directly below the ion source 22 and by disposing itat a predetermined angle relative to the source. In this way, the sizeof the mass spectrometer is considerably reduced and its cost as Well.

Other ion sources, such as the source disclosed and claimed inco-pending application, Serial No. 221,554, filed April 18, 1951 by IanH. McLaren and me, now Pat. No. 2,732,500, may be used as the source 22.In addition, other ion collectors, such as a scintillation counter orelectron multiplier, may be used as the collector 24 as long as they aredisposed in the mass spectrometer in a manner similar to that disclosedabove.

Although this invention has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

What is claimed is:

1. A mass spectrometer, including, means for emitting ions in pulses ina particular direction, means for subjecting the ions to a magneticfield providing lines of flux in a direction substantially perpendicularto the direction of ion emission so as to produce a rotation of theions, means for collecting the ions upon their rotation through asubstantially constant angular distance diiferent from an integralnumber of revolutions, the collecting means being disposed insubstantially the same direction as the magnetic field and at an angleto the emitting means to receive the ions after their angular rotationand means for measuring the times of flight of the ions to determinetheir masses.

2. A mass spectrometer, including, means for providing magnetic lines offlux in a particular direction, means for emitting ions in pulses in adirection perpendicular to the magnetic lines of flux so as to produce asubstantially circular travel path for the ions, means for collectingthe ions after their revolution through a substantially constant angulardistance less than one revolution, the collecting means being disposedin substantially the same direction as the magneitc lines of flux and atan angle to the emitting means to receive the ions after their angularrotation, and means for measuring the time required for the ions ofdifferent mass to rotate through the substantially constant angulardistance.

3. A mass spectrometer, including, an ion source for emitting ions inpulses in a particular direction, magnetic means operative on the ionsin a direction substantially perpendicular to the direction of ionemission to produce a rotation of the ions, a collector assemblydisposed with a plane for recieving the ions in substantially parallelrelationship to the plane of emission of the ions from the ion sourceand in substantially perpendicular relationship to the direction of iontravel, the receiving plane of the collector being disposed inintersecting re lationship to the emitting plane of the ion source tocollect the ions after their rotation through a partial revolution, andmeans for measuring the time required for the ions to reach thecollector assembly.

4. A mass spectrometer, including, an ion source for emitting ions inpulses in a particular direction, magnetic means operative on the ionsin a direction substantially perpendicular to the particular directionto produce a rotation of the ions, a collector assembly disposed insubstantially the particular direction and in intersecting relationshipto the ion source to collect the ions after they have rotated through afraction of a complete revolution, and means for measuring the timerequired for the ions to reach the collector assembly.

5. A mass spectrometer, including, an ion source operative to emit ionsin pulses in a substantially horizontal plane, means magneticallyoperative on the ions to provide for the ions a circular travel path inthe horizontal plane, a collector assembly extending from the ion sourceat substantially a fixed angle relative to the source and inperpendicular relationship to the direction of ion travel and coveringsubstantially the full horizontal and vertical lengths of thespectrometer in directions substantially perpendicular to the directionof ion travel to collect the ions after their rotation through a partialrevolution, and means for measuring the times at which the ions reachthe collector assembly.

6. A mass spectrometer, including, a cover having an axis in a firstdirection, an ion source disposed within the cover in the firstdirection to emit ions in pulses in a direction substantiallyperpendicular to the first direction, magnetic means for producing amagnetic field in substantially the first direction to produce arotation of the ions, a collector extending from the ion source in thefirst direction and across the space Within the cover to collectsubstantially all of the ions after their rotation through asubstantially constant angle constituting a partial revolution, andmeans for indicating the relative times at which the ions of diiferentmass are collected.

7. A mass spectrometer, including, a magnet having a pair of pole piecesseparated from each other by a suitable gap and adapted to produceacross the gap a magnetic field having a substantially uniform density,a cover disposed within the gap and having an axis substantiallyparallel to the magnetic field, an ion source disposed within the coverin the direction of the axis to provide pulses of ions in a directionsubstantially perpendicular to the magnetic field, an ion collectordisposed within the cover adjacent the ion source in the direction ofthe axis and extending across the space within the cover to collectsubstantially all of the ions after their rotation through a partialrevolution, and means for indicating the relative times at which theions of diflerent mass are collected.

8. A mass spectrometer, including, a magnet having a pair of pole piecesseparated by a suitable air gap and adapted to produce across the gap amagnetic flux having a substantially uniform density, a cylindricalcover disposed within the gap with its axis substantially parallel tothe magnetic field, an ion source for emitting pulses of ions in adirection substantially perpendicular to the direction of the magneticflux, an ion collector positioned within the cover relative to the ionsource to collect all of the ions in each pulse after their rotationthrough a substantially constant angle less than a complete revolution,and means for indicating the relative times at which the ions ofdiiferent mass are collected.

References Cited in the file of this patent UNITED STATES PATENTS2,221,467 Bleakney Nov. 12, 1940 2,370,673 Langmuir Mar. 6, 19452,486,199 Nier Oct. 25, 1949 2,582,216 Koppius Jan. 15, 1952

